It is known that the blades constructed of composite materials and currently produced for rotors of rotor aircraft generally include at least one spar, in general consisting of rovings of fibers of high mechanical resistance which are agglomerated by a polymerized synthetic resin and which permit the absorption of the centrifugal forces and of a part of the bending moments which are exerted on the blade in rotation; an external shell which is rigid and profiled to the final profile of the corresponding blade and which is a stress-bearing covering in general consisting of a plurality of layers which are superposed, and preferably crossed, of fabrics of fibers having high mechanical resistance which are rigidified by a polymerized synthetic resin, the covering contributing, with the spar, to withstanding the bending moments and furthermore transmitting the torsion couples which are exerted on the blade; and at least one filling element constructed of a light synthetic material, such as a cellular or foam material, or alternatively a honeycomb material, which is disposed within the stress-bearing external shell in order to fill at least partially the internal volume of the latter and complimentary to that of the spar or spars which likewise traverse this stress-bearing external shell in the longitudinal direction.
In a blade having this conventional structure, the spar, the filling element or elements and the stress-bearing and profiled external shell are the essential elements which participate in the structural resistance of the blade, and which therefore bear the various forces and bending and torsion moments to which the blade is subjected on account of the static, dynamic and aerodynamic loads which are exerted on it, when the rotor of the rotor aircraft is in operation.
The construction of such blades presents a certain number of difficulties, especially on account of the fact that the external shell must, in addition to its aptitude for structural resistance, exhibit at the same time the precise desired aerodynamic profile and likewise good qualities of surface condition, of erosion behavior and of resistance to impacts.
The resistance to impacts and the erosion behavior assume respectively an oversizing of the structural elements and the presence, on the external shell, of at least one supplementary protective layer; this gives rise to a penalty in terms of mass. The obtaining of the precise desired aerodynamic profile and good qualities of surface condition implies, for the manufacture, the utilization of precision equipment, particularly of molds having very low tolerance, and the performance of lengthy and delicate finishing operations, particularly caulkings, fillings, sandings and paintings. This results in a large additional manufacturing cost.
Furthermore, as the covering is structural, it is impossible, in the event of damage, to proceed in situ with simplified repair operations, and it is necessary to demount the blade and to return it to the factory, to restore and then to check the structural integrity of the blade, or to replace it in the event of major damage. It is therefore impossible to repair the profile without repairing the entire blade.
Finally, it is impossible to modify the profile without completely changing the blade in the event, for example, of necessary development of the rotor following a reassessment of the aerodyne.
Moreover, in the field of helicopter rotor blades comprising a trailing edge panel which consists, on the one hand, of a honeycomb web on which are affixed an upper surface skin and a lower surface skin and, on the other hand, which is closed at its internal radial end (turned towards the blade root or towards the hub) by a plug constructed of elastomer, it has been proposed in U.S. Pat. No. 4,335,174 to affix an adhesive band impregnated with a thermosetting resin on the skins and on the edge of the honeycomb web, then to apply an adhesive foam to the adhesive band portion covering the edge of the web, to place the elastomer plug above the foam and the adhesive band, and finally to polymerize the whole by a heat treatment in order that the swelling of the foam should press the adhesive band back against the irregular profile of the edge of the honeycomb web, and that the foam layer should form a regular contour on the web. Possibly, and prior to the rigidification, a second adhesive band which is likewise impregnated is affixed above the foam and the adjacent edges of the first band, and then, after the elastomer plug is placed in position, a third adhesive band is affixed above the first band and the edges of the elastomer which cover this first band. However, these means permit solely the obtaining of a joint which is perfectly sealed between the elastomer plug and the trailing edge panel, by virtue of the foam and the rigidified adhesive band or bands.